Plasma Ultrafiltrable Calcium Concentration in Posthypercapnic Ventricular Fibrillation

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Abstract
During four hours of severe respiratory acidosis produced by inhalation of 30% and 40% CO2 in dogs, plasma potassium and inorganic phosphate rose, ultrafiltrable calcium first increased then fell below control level, and total calcium increased slightly. Within 5 minutes after returning the animal to air breathing, potassium showed a further sharp increase, ultrafiltrable calcium a further sharp decrease and total calcium and inorganic phosphate showed no change. With the rise in potassium and fall in ultrafiltrable calcium the UF Ca/K ratio fell to less than one third control value. This appears to be an important factor in the mechanism producing cardiac arrhythmias and ventricular fibrillation in the immediate post-hypercapnic period. In vitro experiments on dog blood indicate that ultrafiltrable calcium varies directly with H+ concentration as expected, and that an increase in inorganic phosphate produces a decrease in ultrafiltrable calcium. Operation of these two factors helps to explain the change in plasma ultrafiltrable calcium observed in the in vivo experiments. It is suggested that the decrease in ultrafiltrable calcium in the presence of the very high phosphate concentrations observed, may be due to the formation of some nonfiltrable calcium phosphate compound. It is possible that the increased phosphate concentration increases the binding of calcium in some other form.

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